After exchanging a few letters with someone new (let's say 10k words total), one could have the option of rating this pen pal as someone who likely doesn't use AI to write their letters. If someone is rated by 10 people, they get a "Possibly not AI-generated" stamp. If they are rated by 20, "Probably not AI-generated," and so on, until 50 ratings. They can put those stamps as their favorite in their profile, so people feel more confident in exchanging letters with that person.